This invention relates to optical recording media for use with optical systems wherein a focussed laser beam mechanically scans the recording medium to either write or read information thereto or therefrom. Such optical recording media is typically, but not necessarily, in the form of a disc. However, for concreteness, the invention will be presented within the context of what is commonly known as "optical disc redording."
In optical recording it is desirable to have a recording medium that is sensitive to a source of optical energy, such as a laser beam, yet is inherently protected from dust particles, as well as any other forms of particles or undesirable conditions resulting from the manual handling environment to which the media must be subjected. In addition, it is desirable to have pilot tracks on the unrecorded disc for purposes of developing a signal for track-following servos. It is also desirable, in order to achieve good sensitivity in many types of optical recording media, to leave a free surface on one side of an active layer, the "active layer" being that layer in which the laser energy "writes" information by selectively changing the optical characteristics, or otherwise marking, of the layer. A free surface is desirable because many recording processes are essentially thermal in nature, and the recorded mark is produced when the temperature of an active layer is elevated to some critical temperature by the focused laser recording beam. For example, a typical marking mechanism is the perferation of, or "hole forming" in, the active layer. In such thermal recording processes, a key characteristic limiting the sensitivity of the media is the thermal diffusion of energy away from the local spot to be heated. Thus, the thermal confinement of this energy to the region of the spot to be recorded is favored by substrates having low values of thermal diffusivity and by structures in which the active layer has a free surface, since the thermal diffusivity into air, vacuum, or gas is very much less than even into the best insulating solid materials.
The prior art has attempted to meet the objectives stated above by providing dust and handling protection through the use of a thick transparent window such that the external surface upon which the dust collects is always out of focus to the optics used in connection with the laser beam. The use of an active layer having a free surface in order to improve the sensitivity thereof is also known in the art. Both of these features are disclosed, for example, in U.S. Pat. No. 4,074,282. Other patents known to the applicants that provide a similar window to protect a free surface of an active layer include U.S. Pat. Nos. 4,264,911 and 4,308,545.
There are, however, several problems associated with the configurations shown in the above-referenced patents, for which it is the objective of this invention to overcome. For example, if the window is made of glass it is subject to catastrophic failure during handling or during operation when the disc might be rotating at speeds in excess of 2000 rpm. On the other hand, if the window is made of plastic, it is subject to plastic creep due to the high centrifugal stresses that are involved in speeds in excess of 2000 rpm. Furthermore, since the window typically spans a radial distance between supports of two or three inches, the window must be significantly thick not to be plagued by vibrating modes of the "drumhead" type. In addition, the dimensional tolerances of windows, particularly thick windows, become very stringent for systems using high numerical apperature focusing objectives. That is, it is difficult to obtain transparent optical plastics meeting the required tolerances on thickness variations at thicknesses sufficient to be rigidly self supporting.